A wide variety of processes for producing purified terephthalic acid (PTA) have been disclosed in the prior art. However, only a handful of these prior processes are widely practiced commercially. One such commercial process employs two stages of oxidation, with liquor exchange between the oxidation stages. In the first stage of oxidation, referred to herein as “primary oxidation,” para-xylene is oxidized to terephthalic acid (TPA). The product of primary oxidation is a crude slurry containing a liquid mother liquor and crude terephthalic acid (CTA) particles. This crude slurry produced in primary oxidation is subjected to a liquor exchange process that replaces a substantial portion of the original mother liquor with a cleaner solvent. The resulting liquor-exchanged slurry is then purified in the second stage of oxidation, referred to herein as “oxidative digestion.” Oxidative digestion produces purer TPA particles through a process that involves the continuous dissolution and reprecipitation of TPA particles under oxidation conditions. The TPA particles produced from oxidative digestion are purer than the CTA particles introduced into oxidative digestion for two main reasons: (1) reaction intermediates (e.g., 4-carboxybenzaldehyde (4-CBA) and para-toluic acid (PTAC)) originally trapped in the CTA particles are further oxidized to TPA during oxidative digestion; and (2) the dissolution and reprecipitation associated with oxidative digestion partitions a portion of the relatively unreactive aromatic impurities (e.g. isophthalic acid (IPA)) out of the solid phase and into the liquid phase. In addition to increasing the purity of the TPA particles, oxidative digestion also has the advantage of producing TPA particles that are larger than the CTA particles subjected to oxidative digestion. These larger TPA particles produced by oxidative digestion facilitate more efficient and effective downstream processing.
The liquor exchange step between primary oxidation and oxidative digestion serves two main functions: (1) removal of soluble, relatively unreactive aromatic impurities (e.g., IPA) from the solid CTA; and (2) removal of catalyst compounds present in the liquid phase of the crude slurry. The removal of relatively unreactive aromatic impurities provided by liquor exchange allows the CTA to be adequately purified without hydrogenation, which is very expensive. The catalyst removal provided by liquor exchange reduces chemical activity during oxidative digestion, leading to reduced carbon burn losses while still retaining reactivity necessary for further conversion of aromatic reaction intermediate compounds to TPA. The reduction of catalyst concentrations provided by liquor exchange also makes removal of catalyst compounds more efficient and more complete during subsequent isolation of solid PTA product.
Although liquor exchange between the primary oxidation and oxidative digestion steps has its advantages, it can be expensive and difficult to continuously remove the hot, flammable, corrosive, mother liquor from the crude slurry and continuously replace the removed mother liquor with the hot, flammable, corrosive, cleaner solvent. A particularly significant expense associated with this type of liquor exchange step is the liquor exchange that typically takes place in one or more large centrifuges or pressure filters made of expensive metals (e.g., titanium) or metal alloys.
In the past, several sources have proposed that PTA could be made without employing a liquor exchange step between primary oxidation and oxidative digestion. However, in such proposed systems, the increased catalyst concentrations in the feed to oxidative digestion dramatically increases carbon burn losses associated with oxidative digestion. In addition, the proposed PTA production systems that eliminate liquor exchange between primary oxidation and oxidative digestion typically employ a liquor exchange step downstream of oxidative digestion. In this type of system, the mother liquor removed downstream of oxidative digestion has a higher concentration of relatively unreactive aromatic impurities (e.g., IPA) than the mother liquor upstream of the second stage of oxidation. This is because oxidative digestion increases partitioning of relatively unreactive aromatic impurities into the liquid phase. In a continuous PTA production process employing recycled solvent (i.e., recovered and purified solvent originating from mother liquor produced from primary oxidation) as a feed to primary oxidation, the relatively unreactive aromatic impurities not exiting with solid PTA product accumulate in the recycled solvent until otherwise removed or destroyed. Unless auxiliary process steps for purification of the recycled solvent are increased in scope, the concentrations of relatively unreactive aromatic impurities (e.g., IPA) in the recycled solvent continue to rise over time, setting off a cascade of chemical and process consequences such as, for example, an undesirable increase in the formation rate of colored aromatic impurities in primary oxidation and an eventual increase in the color of solid TPA product. The particulars of auxiliary process steps for purification of the recycled solvent have a number of complex interactions with the primary oxidation and oxidative digestion steps and can influence operating costs and product quality significantly. For example, increased recycle of uncolored IPA will actually increase the formation rate of highly colored 2,7dicarboxyfluorenone (2,7-DCF) with considerable eventual adverse affect on solid TPA product color as the levels of IPA and 2,7-DCF slowly rise to a new steady state concentrations throughout the process.
In view of the foregoing, prior art processes employing two stage oxidation without intermediate liquor exchange have not proven to be commercially viable because, for example, (1) they exhibit increased carbon burn losses during oxidative digestion, (2) they can not use recycled solvent, and/or (3) if recycled solvent is used, they require additional expensive purification systems to control the increased contaminant levels in the recycled solvent.